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Estimated quantities of proved reserves (Details 4) | 12 Months Ended
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Dec. 31, 2013 MBoe | Dec. 31, 2012 MBoe | Dec. 31, 2011 MBoe
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year, BOE | 80,486 | 97,651 | 107,599
Production, BOE | (10,264) | (10,058) | (11,099)
Extensions and discoveries, BOE | 19,050 | 14,756 | 10,842
Improved recovery, BOE | 0 | 0 | 0
Purchases of proved reserves, BOE | 0 | 0 | 247
Sales revisions, BOE | (7,987) | (423) | 0
Revisions of previous estimate, BOE | (590) | (21,440) | (9,938)
Balance at end of year, BOE | 80,695 | 80,486 | 97,651
Reduction in proved undeveolped reserves specified properties |  |  | 8,900
Oil (MBbls) |  |  |
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year | 33,453 | 27,005 | 25,666
Production | (4,815) | (3,694) | (2,724)
Extensions and discoveries | 13,313 | 9,874 | 4,717
Improved recovery | 0 | 0 | 0
Purchases of proved reserves | 0 | 0 | 223
Sales of proved reserves | (1,286) | (39) | 0
Revisions of previous estimates | 354 | 307 | (877)
Balance at end of year | 41,019 | 33,453 | 27,005
NGL (MBbls) |  |  |
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year | 7,153 | 7,342 | 7,201
Production | (781) | (828) | (776)
Extensions and discoveries | 1,333 | 1,817 | 1,421
Improved recovery | 0 | 0 | 0
Purchases of proved reserves | 0 | 0 | 16
Sales of proved reserves | (25) | 0 | 0
Revisions of previous estimates | (1,078) | (1,178) | (520)
Balance at end of year | 6,602 | 7,153 | 7,342
Natural Gas (MMcf) |  |  |
Proved developed and undeveloped reserves: |  |  |
Balance at beginning of year | 239,278 | 379,827 | 448,397
Production | (28,008) | (33,214) | (45,598)
Extensions and discoveries | 26,428 | 18,386 | 28,221
Improved recovery | 0 | 0 | 0
Purchases of proved reserves | 0 | 0 | 54
Sales of proved reserves | (40,055) | (2,307) | 0
Revisions of previous estimates | 802 | (123,414) | (51,247)
Balance at end of year | 198,445 | 239,278 | 379,827

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- Definition Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields, BOE. + References No definition available. + Details
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- Definition Changes in proved reserve estimates resulting from application of improved recovery techniques, BOE. + References No definition available. + Details
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- Definition The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable, BOE. + References No definition available. + Details
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- Definition Production of proved reserves, BOE. + References No definition available. + Details
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- Definition Purchase of minerals in place, BOE. + References No definition available. + Details
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- Definition Revisions represent changes in previous estimates of proved reserves, either upward or downward, resulting from new information (except for an increase in proved acreage) normally obtained from development drilling and production history or resulting from change in economic factors, BOE. + References No definition available. + Details
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- Definition Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447 + Details
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- Definition The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-10.(a)(2)) -URI http://asc.fasb.org/extlink&oid=27015464&loc=d3e511914-122862 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 10 -Paragraph a -Subparagraph 2 -Article 4 + Details
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